The present invention relates to an optoacoustic gas sensor and, more particularly, to an optoacoustic gas sensor with a sensor body having a light source, a measurement cell with a gas-permeable membrane and a measurement microphone, and evaluation electronics.
Such gas sensors operate on the basis of the photoacoustic effect, whereby modulated-light irradiation of a gas to be detected gives rise to an acoustic pressure wave whose magnitude is directly related to the concentration of the gas. The acoustic pressure wave arises as the gas absorbs the optical radiation and heats up and expands as a result. The pressure fluctuations correspond to the modulation of the optical radiation.
From the measured acoustic pressure, the gas concentration can be inferred. Different gases are distinguished by use of light sources having different wavelengths corresponding to specific absorption wavelengths of the gases. Laser sources, or broadband light sources such as coiled filaments together with band-pass filters, can be used for this purpose. Gas sensors of this are described in European patent applications EP-A-0 760 474 and EP-A-0 798 552, and in respectively corresponding U.S. patent application Ser. No. 08/706,240 of Sep. 4, 1996 and Ser. No. 08/828,837 of Mar. 24, 1997 which are incorporated herein by reference.
Preferred gas sensors have a measurement cell whose longitudinal axis extends perpendicular to the longitudinal axis of the sensor body, and have a light source which is disposed to irradiate the measurement cell without irradiating the membrane, thus to minimize interference signals. Also, such gas sensors are explosion-proof, as the light source is sealed off from the ambient atmosphere. This type of photoacoustic sensor has proven effective in use, at least so long as the concentration of the gases to be detected lies above a certain minimum concentration, which for CO.sub.2 is practically always the case. But combustible solvents containing CH bonds in the range from 300 to 3,000 ppm, or NH.sub.3 in the range from 100 to 200 ppm are not readily detectable with such a sensor.
Potentially, the detection of combustible solvents containing CH bonds is a particularly important application for optoacoustic gas sensors, as pelliators can be used only to a limited extent in the range from 300 to 3,000 ppm, and can become contaminated easily. Metal-oxide sensors would be suitable, but they suffer from instability.
The sensitivity of optoacoustic gas sensors is limited due to interference signals caused by wall effects (zero signal), fluctuations in air pressure (caused by the actuation of doors or of ventilation systems), and vibrations (of the building or due to motors or persons). An immediate remedy for minimizing the effect of interference signals lies in the use of larger measurement cells and larger light sources. However, this would result in an appreciable increase in the dimensions of the sensor body and to a corresponding increase in sensor costs.